Cu—Zn—Al Shape Memory Effect (SME) alloys are promising smart and intelligent engineering materials. (Wayman C. M., Journal of Metals, 32 (June 1980), p. 129–137 and Michael A. D & Hart W. B Metal Material Technology, 12(1980), p. 434–440. These have attracted much attention because of their low cost and ease of fabrication relative to nitinol (White S. M., Cook J. M. & Stobbs W. M, Journal De Physique, C4 (ICOMAT-82), P-779–783. But nitinol has superior properties, long fatigue life and is biocompatible.
There are about twenty elements in the central part of the periodic table Golestaneh A. A., Physics Today, (April 1984), p-62–70 whose alloys exhibit shape memory like Ag—Cd, Au—Cd, Cu—Al—Ni, Cu—Al—Mn, Cu—Au—Zn, Cu—Sn, Cu—Au—Sn, Cu—Zn, Cu—Zn—Al, Cu—Zn—Sn, Cu—Zn—Ga, Cu—Zn—Si, In—Ti, Ni—Al, Ni—Ti, Fe—Pt, Fe—Pd, etc. (Wayman C. M., Journal of Metals, 32 (June 1980), p-129–137 and Michael A. D & Hart W. B Metal Material Technol., 12(1980), p. 434–440.
Shape memory alloys (SMA) have a unique property, i.e., these materials remember their past shapes/ configurations. The Important characteristics of these alloys are their ability to exist in two distinct shapes or configurations above or below a certain critical transformation temperature. It undergoes diffusionless martensitic transformation Golestaneh A. A., Physics Today; (April 1984), p. 62–70, which is also thermo elastic in nature, i.e., below the critical temperature a martensitic structure forms and grows as the temperature is lowered, whereas, on heating the martensite shrinks and ultimately vanishes.
The martensite in shape memory alloys is soft in contrast to martensite of steels. Deformation of these alloys is not by slip, twinning or grain boundary sliding but by growth or shrinkage of self-accommodating, multi-oriented martensitic plates/variant Saburi T., Wayman C. M., Takala K & Nenno S., Acta Metallurgica (January 1980) P-15.
On heating, the strained martensite reverts back to its parent phase, thereby, the original undeformed shape is recovered. The change in structure can be linked with change in shape and dimensions and the alloy exhibits a memory of high and low temperature shapes. There is a usable force associated with these shape changes and thus the alloys can be incorporated into range of temperature sensitive devices for warning, control, detection, regulation etc. The actuators can be calibrated to operate within a narrow temperature range by incorporating a compensating bias spring. The recoverable strain is 2–8% and is dependent upon one or two way memory. Copper based shape memory alloys in addition to one-way memory also exhibit two-way memory behavior, after undergoing a suitable thermal-mechanical processing called training (Wayman C. M., Journal of Metals, 32 (June 1980), p. 129–137 and Michael A. D & Hart W. B Metal Material Technol., 12(1980), p. 434–440.
Once trained, the material will spontaneously change its shape when heated or cooled, above or below the respective transformation temperatures. Forward and reverse martensitic transformation temperatures are designated as ‘Ms’ (while cooling) and ‘As’ (while heating) respectively.
In Cu—Zn—Al ternary alloys, shape memory effect lies in the Copper rich corner of the triangle in the form of a trapezium. On enlarging this trapezium we can correlate composition with martensitic transformation temperature Schetky L. M., Scientific American, 241 (November 1979), p. 68–76.
The amount of Aluminum varies from 4–10%, Zinc from 10–30% and balance is Copper. Martensite formation temperature (‘Ms’) varies from −100° C. to +300° C. as a result of very small change in composition. But useful range for Aluminum brass is −70° C. to +150° C.
Martensitic transformation temperature (As) is extremely sensitive to composition. A slight variation of either of the elements, Zinc or Aluminum (say ±0.5%) shifts the transformation temperature by ±50° C. Therefore close control of composition is utmost essential to get the desired transformation temperature for the actuator to work at a specific temperature. Loss of low melting and volatile elements like Al, Zn etc. while melting cannot be avoided in air melting furnaces. Vacuum melting furnaces, in which close control of composition is possible but their installation is extremely costly and are unaffordable to the small and medium scale melting units/industries.
In air melting furnaces, there is always a danger of loss of such elements in spite of compensating these losses and following the necessary precautions rigidly during melting. The alloy with off-composition and undesired martensitic transformation temperature has to be rejected or remelted. The efforts and inputs, thus put in, go waste. It was also observed that loss of zinc or aluminum raises the martensitic transformation temperature whereas increase of these elements lowers the transformation temperature.
Hence, the present invention is directed towards increasing or decreasing of martensitic transformation temperature.
In Cu—Zn-4% Al alloy Adnyana D. N., Wire Journal International, (1984), pp. 52–61, lowering of martensitic transformation temperature has been comparatively low, i.e., around 20° C.–25° C.
There is always evaporation of volatile and low melting elements like zinc, aluminum, tin, lead etc during the melting of copper base and other alloys, especially in the air melting furnaces. These losses cannot be avoided but can be minimized by taking all the care during melting, adding precisely weighed quantities of each element, compensating for the elemental losses and rigidly following precautions during melting.
Vacuum furnaces precisely control these losses but their installations are costly and are thus unaffordable to the small and medium scale melting/foundry units. Cu—Zn—Al shape memory alloys (SMAs) are no exceptions to these. The martensitic transformation temperature (As) is an important parameter in shape memory alloys and is extremely sensitive to the composition. A slight variation of either zinc or aluminum (±0.5%), as a result of melting losses, shifts the martensitic transformation temperature by ±50° C. The material thus cast and processed reduces to a scrap and has to be remelted thereby resulting in wastage of efforts, manpower and machinery.
Experimental studies show that it is possible to raise As temperature by 15° C.–20° C. by the use of either a compensating bias spring or by selective etching/leaching out of zinc by thermal treatments. But lowering of As temperature, once obtained poses problems.
U.S. Pat. No. 4,634,477 recites about shape memory alloys. However, this patent does not mention about the reduction in martensitic temperature.